1. Field of the Invention
The present invention relates to boost converter circuits with reduced switching losses.
2. Discussion of the Related Art
Generally, at higher power levels, the continuous-conduction-mode boost converter is the preferred topology for implementing a front-end converter for active input-current shaping. The output voltage of a boost input-current shaper is relatively high since the dc-output voltage of the boost converter must be higher than the peak input voltage. Because of this high output voltage, the boost converter requires a fast-recovery rectifier. At high switching frequencies, a fast-recovery rectifier produces significant reverse-recovery-related losses when switched under "hard" switching conditions. As a result, to avoid a significant deterioration of conversion efficiency, a hard-switched, boost input-current shaper operates only at relatively low switching frequencies. Soft-switching techniques are developed to increase the switching frequency and the power-density of the front-end boost converter.
A soft-switched boost converter typically uses an active snubber consisting of an auxiliary active switch with a few passive components (inductors and capacitors). The active snubber controls the rate of change of the rectifier current ("di/dt") and creates soft-switching conditions (e.g., zero-voltage switching (ZVS) conditions) for the boost switch and the rectifier.
Some examples of soft-switched boost converters can be found in (i) "High efficiency telecom rectifier using a novel soft-switched boost-based input current shaper," by R. Streit and D. Tollik, International Telecommunication Energy Conf. (INTELEC) Proc., pp. 720-726, October 1991; and (ii) "Novel zero-voltage-transition PWM converters," by G. Hua et al., IEEE Power Electronics Specialists' Conf. (PESC) Rec., pp. 55-61, June 1992. In these examples, a snubber inductor connected to the common node of the boost switch and the rectifier controls the rate of change of the rectifier's current. Because of the snubber inductor's location, the boost switch and the rectifier experience minimum voltage and current stresses. In addition, the boost switch and the rectifier turns on and turns off, respectively, under ZVS conditions. In these examples, however, the auxiliary switch operates under hard-switching conditions, being turned on while its voltage is equal to the output voltage, and being subsequently turned off while carrying a current greater than the input current.
In these circuits disclosed by Streit and Tollik, and Hua et al., a severe resonance can occur between the output capacitance (C.sub.oss) of the auxiliary switch and the resonant inductor, after the auxiliary switch is open and the snubber inductor current falls to zero. This resonance adversely affects the proper operation of the circuit and must be eliminated. To minimize this resonance, Hua et al. provide an additional rectifier and a saturable inductor in series with the snubber inductor. However, the additional components degrade conversion efficiency and increase the cost of the circuit.
Other examples of soft-switching boost converters can be found in (i) "New, zero voltage switching, high frequency boost converter topology for power factor correction," by J. Bassett, International Telecommunication Energy Conf. (INTELEC) Proc., pp. 813-820, 1995; (ii) "A new family of ZVS-PWM active-clamping dc-to-dc boost converters: analysis, design, and experimentation," by C. M. C. Duarte et al., International Telecommunication Energy Conf. (INTELEC) Proc., pp. 305-312, 1996; and (iii) "A technique for reducing rectifier reverse-recovery-related losses in high-voltage, high-power boost converters," by M. Jovanovic, IEEE Applied Power Electronics (APEC) Conf. Proc., pp. 1000-1007, 1997. In these examples, the rate of change of the rectifier current is controlled by a snubber inductor connected in series with the boost switch and the rectifier. Because of this snubber inductor's location, the voltage stress of the boost switch is higher than that of the boost switches in the circuits described by Streit and Tollik, and Hua et al. discussed above. This increased voltage stress can be minimized by carefully selecting both the inductance value for the snubber inductor and the switching frequency. For properly designed converters, the boost and the auxiliary switches in the circuits of Bassett, Duarte et al., and Jovanovic operate under ZVS conditions.
An example of such boost converters is boost converter 100 shown in FIG. 1, which is described in the Jovanovic reference mentioned above. As shown in FIG. 1, boost converter 100 includes boost inductor 102, boost switch 106, boost rectifier 109, and output filter capacitor 111 configured in a conventional boost converter configuration. The input and output ports of boost converter 100 are respectively an input voltage source 101 and a resistor 112 representing the load of circuit 100. In addition, an inductor 104 is provided in series connection with boost switch 106, between boost switch 106 and the common node 114 of boost inductor 102 and boost rectifier 109. An auxiliary switch 107 is provided to couple a clamping capacitor 110 between output node 113 and boost switch 106. Two anti-parallel diodes (internal body diodes of the MOSFETs), represented in FIG. 1 as rectifiers 105 and 108, are provided across boost switch 106 and auxiliary switch 107, respectively. An additional rectifier 103 is provided to clamp common node 114 to the lower supply voltage at node 115.
These circuits described by Bassett, Duarte et al., and Jovanovic, such as boost converter 100 above, require either (i) an isolated (high-side) gate drive, if the auxiliary switch is an N-channel MOSFET, or (ii) a P-channel MOSFET, if a non-isolated (direct, low-side) drive is to be used. Either choice, i.e., an isolated gate drive or a P-channel MOSFET, increases circuit complexity and cost. Furthermore, both Duarte's and Jovanovic's circuits require precise and noise-robust gate-drive timing, as accidental overlapping of the main and auxiliary switch gate drives may result in a catastrophic failure due to the large transient current in the main and auxiliary switches simultaneously. Furthermore, the Duarte's circuit also suffers from a parasitic resonance between the rectifier's junction capacitance and the snubber inductor, which significantly increases the voltage stress on the rectifier. Thus, Duarte's circuit requires a rectifier with a higher voltage rating, which further increases the cost of the circuit and reduces its conversion efficiency.